Hi Peter,
This is a known issue ( not a Tool Shed issue, but a Galaxy issue ) related to
the Select list that gets automatically populated with the "Select one" option.
You have to actually click the x" on this option to eliminate it. Again, this
feature is inherited from Galaxy, so it's not specific to the tool shed. I
haven't had time to track it down and fix it since I'm under water with just
the tool shed.